The present invention relates to a rubber composition for coating a steel cord and a steel cord coated thereby.
Durability at high pressure and high strain and more adhesiveness with a steel cord have been recently required for a rubber composition for coating a steel cord of a bus or truck tire in accordance with increase of a loading of a bus or truck tire and the JATMA approving air pressure.
As a process of improving adhesiveness with a steel cord, it is known that a cobalt salt, silica, a resin and the like are compounded in a rubber composition for coating a steel cord. However, even if these are compounded, adhesiveness with a steel cord is insufficient and there was a problem that the rubber composition is peeled from the steel cord due to running.
Also, as a process of improving adhesiveness with a steel cord, it is known that a large quantity of sulfur is compounded. However, free sulfur which is not bonded with a rubber or a steel cord is increased in the vulcanized rubber composition, the sulfur lowers tensile properties (strength and elongation at break) of the rubber composition by deterioration caused by heat and oxygen during using the tire, and there has been a problem that cohesive failure in the rubber composition is generated in a durability test.
Japanese Examined Patent Publication No.7-15029 discloses technology by which adhesiveness with a steel cord is improved by using the specified amount of a partial condensate of hexamethylolmelamine pentamethyl ether, a methacresol resin, sulfur and cobalt naphthenate. However, due to using a large amount of cobalt naphthenate, physical properties such as strength at break are lowered and the methacresol resin tends to inhibit a sulfur crosslinking reaction at vulcanization; therefore rubber vulcanization is slow and there was a problem that rubber flow around a steel cord is increased by shaping pressure. Accordingly, it was difficult to homogeneously integrate the steel cord with a rubber.